This invention relates to a vacuum lamination station and more particularly to a vacuum bag lamination material and a method for using such a material for the vacuum lamination of printed circuit boards to a heat sink.
Vacuum lamination is known in the art. Processes employing vacuum bags are presently being used to laminate many items from skis to advanced composite aircraft wings. Vacuum lamination is a method of applying a uniform pressure over a flat or curved object which may be difficult or impossible to achieve using rigid presses.
In the manufacturing of certain printed circuit board (PCB) assemblies, a single sided surface mount PCB is bonded to a heat dissipation element or heat sink using a heat curable adhesive. This bonding process is usually done in a heated press. The heated press provides both the heat required to cure the adhesive and the pressure required for intimate contact of the PCB and the heat sink at the bond line. A PCB with electrical components attached to it is called a populated PCB and the side of the PCB where electrical components are placed is known as the component side. In the past only unpopulated circuit boards could be bonded to heat sinks because the heated press would crush any components on the PCB. The required testing of a circuit board for component polarity and for electrical shorts or opens between component leads and the pads that they are soldered to can only be done on populated circuit boards. This testing is greatly facilitated if nodes on the bottom (heat sink side) of the circuit board can be accessed by test equipment probes. If the circuit board is laminated to a heat sink, these nodes are inaccessible and the testing is difficult and time consuming. Since the components could not be mounted on the board prior to lamination, it was extremely difficult to test the circuit board after lamination. In some instances, an irreparable fault was found during testing and, since the bad board could not be removed from the heat sink, the whole assembly would have to be scrapped.
By using a vacuum lamination system, curing of adhesives may be carried out on populated PCB assemblies which, due to the presence of electrical components, have irregular surfaces. In this method, after the adhesive is applied between the circuit board and heat sink, the unit is placed in a bag, and a vacuum is established inside the bag. The pressure differential between the inside and outside of the bag maintains the required contact pressure between the board and heat sink during an adhesive heat-curing operation.
However, the commercially available bag materials used for vacuum lamination are made of plastic films. One disadvantage of these films is that they are very inelastic. In this regard they are best suited for applications involving smooth, gradual contours because the inelasticity of the material causes severe "tenting" around abrupt surface features such as electrical components. Tenting creates an uneven pressure distribution which is unacceptable for laminating populated printed circuit boards to heat sinks.
Another disadvantage of plastic film bags is that they become stiff and brittle after the heat curing process which can result in tearing of the bags. They are thus best suited to one-time-use situations.
Some vacuum lamination methods have employed silicon rubber sheets as the vacuum bag material. However, both the plastic films and the silicon rubber sheets have smooth surfaces which will seal against each other around the periphery of the PCB assembly and prevent the vacuum from spreading throughout the bag. This prevents the even pressure distribution required for proper lamination. In typical vacuum lamination processes, a breather cloth is usually placed between the unit to be laminated and the inside surface of the bag to provide a pathway for air to escape during air evacuation. But, the breather cloth increases the tenting problem and can become enmeshed in and bonded to any adhesive that flows from the bond line.
An additional problem with vacuum bags made of either plastic or silicon rubber is that the bag materials build up static charges as the inside surfaces press against each other during air evacuation. These charges can be greater than 10,000 volts. Since many electronic components are highly sensitive to electrostatic discharge (ESD) and can be damaged by charges as low as 50 volts, many of the components on the PCB would be destroyed. Antistatic materials and treatments were found to be ineffective when subjected to the lamination process.
It is therefore an object of the present invention to provide a lamination material which overcomes the disadvantages of the prior art.
It is an additional object of the present invention to provide a lamination material which is easy to use and cost effective.
It is yet another object to provide a method of vacuum laminating populated printed circuit boards to heat sinks.
These objects and others which will become apparent hereinafter are accomplished by the present invention which provides a sheet material for use in a process for laminating an article. The sheet material includes an elastic material substance, an electrically conductive material incorporated into said elastic material, and means located on a surface of the elastic sheet for permitting air to traverse the surface when the sheet is in contact with a smooth surface.
The present invention also provides a method of laminating a printed circuit board including providing a printed circuit board having two opposed surfaces, positioning an electronic component on one surface of the board, applying an adhesive to the other surface of the board, positioning a heat sink adjacent the adhesive; placing a sheet of flexible, electrically conductive material surrounding the board and heat sink to form an enclosure, the sheet having means located on an area thereof such that air is permitted to move along the area when the sheet is in contact with the one surface of the board; establishing a vacuum in the enclosure; curing the adhesive to form a laminated assembly; and removing the laminated assembly from the vacuum enclosure.